Xerographic printing has evolved along the lines of higher throughput and increasing print quality. Current attempts at increasing throughput have focused on the use of multiple laser beams to concurrently image a photoreceptor surface. The throughput of a xerographic printing system is usually proportional to the number of independently addressable beams imaging the surface.
One known way of achieving high throughput is by using arrays of laser diodes. Multiple, independently addressable diodes allow for parallel writing to a photoreceptor; thereby increasing throughput. While multiple laser beams promise higher throughput, their use does not necessarily guarantee better print quality.
Improved print quality has generally been a function of resolution which, in turn, depends upon spot size and the accurate spacing of spots. As a general rule, a smaller spot size allows higher resolution. Likewise, the resolution is increased the closer and more accurately the spots can be spaced.
Typically, the spacing of beams generally takes place in two orthogonal axes: tangential and sagittal. The tangential plane in a raster output scanner (ROS) system is generally the top view as seen from the axis of rotation of the mirrored polygon. The sagittal view is generally the side view as seen from a single mirrored facet of the polygon. A good discussion of the tangential and sagittal planes is found in "Laser Scanning for Electronic Printing" by Urbach et al. as published in the Proceedings of the IEEE, Vol. 70, No. 6, June 1982, which is herein incorporated by reference.
Laser diode arrays are generally of two different varieties: monolithic and nonmonolithic. Monolithic arrays of laser diodes are arrays of laser stripes (i.e. layers of materials that laser when an electric current runs through them) that are produced as a unitary structure in the manufacturing process. By contrast, nonmonolithic arrays are structures that are not constructed as unitary arrays. Instead, a nonmonolithic array usually comprises a separate submount and a plurality of laser diodes that are coupled to the submount in some fashion such as solder, epoxy, or the like.
For nonmonolithic arrays of laser diodes, accurate spacing along the tangential axis can be controlled by the precise construction of a submount on which the laser diodes are coupled. The construction and structure of such submount units are described in greater detail in commonly assigned U.S. Patent Application Number (as yet unassigned), entitled "Nonmonolithic Arrays of Accurately Positioned Diode Lasers", filed on (as yet to be filed) by Biegelsen et al. (Attorney Docket Number D/93080) which is herein incorporated by reference. Accurate tangential spacing can eliminate the registration error of two beams writing across the same scan line, as is desirable in color xerography.
For monolithic arrays, accurate spacing along the tangential axis can be controlled by the present techniques of semiconductor fabrication such as epitaxial deposition, photolithography, ion implantation, and the like. It will be appreciated that these techniques are very well known in the art.
Accurate spacing of beams in the sagittal direction is perhaps even more important to the human eye. Errors in the sagittal plane show up as uneven spacing between adjacent scan lines. These errors are generally noticeable to the naked eye; and, consequently, decrease the perceived print quality.
Thus, there is a need to accurately control the spacing between laser diode beams in the sagittal plane to improve print quality.
It is therefore an object of the present invention to provide several methods for controlling the spacing of laser diode beams along the sagittal axis.